Home-Built Relay Computer Clicks & Clacks on the Wall

While the rest of the world may be focusing on miniaturization, Harry Porter is headed in the opposite direction. A professor in computer sciences, Porter has a strong interest in showing just exactly how these systems work. To that end, he's built a very impressive relay computer, which occupies a place (make that a big place) of honor on his living room wall.

Harry proudly displaying his relay computer.

Porter's relay computer consists of four physical units: arithmetic logic, register, program control, and a sequencer unit. Each is housed in a nice wooden frame with a glass front for display. Everything is organized logically and LEDs are in place so you can actually see the data flow through the system while the 415 total relays emit a familiar and satisfying cacophony. You can see and hear it in the video below.

The specs, taken from Harry's documentation, are as follows:

General specs

Data Bus (8 bits)

Address Bus (16 bits)

All relays are identical (Four-Pole-Double-Throw, 12 Volts)

415 Relays

111 Switches

350 LEDs

Max Power Consumption: Estimated 12 Amps @ 13.5 Volts (160 Watts)

Arithmetic Logic Unit

Two 8-bit inputs (from B and C registers)

8-bit result (onto data bus)

3-bit function code input

Functions: Add, Increment, And, Or, Xor, Not, Shift-left, Nop

Carry output (from Add, Increment)

Zero-detect output

Register Unit

8 registers (8 bits each)

Register Names: A, B, C, D, M1, M2, X, and Y

Data Bus (8 LEDs and 8 Switches)

Program Control Unit

Program Counter (16 bits)

Instruction Register (8 bits)

Jump Target Register (16 bits)

Increment Unit (16 bits)

Increment Register (16 bits)

Address Bus (16 LEDs and 16 Switches)

Sequencer Unit

Clock (Using capacitors for delay)

Finite State Control (24 states)

Instruction Decoding

Main Memory (32K x 8 bits, static RAM chip)

Porter completed the computer in 2007. When I asked him if it still worked, he replied "Yes, it is still functional. But it doesn't get much use. I tend to read email on my iPad instead," which is completely understandable. It now waits on standby, ready to be powered on if an excuse presents itself.

Registers and switches.

Click the image above to view the entire slideshow of 17 images.

Programming is quite an arduous task that involves first selecting an address, then using switches to select the byte you wish to enter and then push that into the memory. Repeat this process over and over until you're ready to begin your computation. There are a few sample programs documented on Porter's site that demonstrate how the machine does simple addition, subtraction, and multiplication.

Harry has done a fantastic job of documenting the build. He shares not only a wonderful set of pictures of the process, but also PowerPoint presentations, schematics, and a 60-minute detailed breakdown of the relay computer's design.

First, thanks for putting "clicks and clacks" in the headline. It's hard for me to resist any story with that! I LOVE this project. It is not just a great demonstration of how a computer works, but it is true kinetic art. It belongs in a museum like the Smithsonian, which I'm relatively sure would love it.

(I recently visited the Smithsonian and, to my astonishment, there was a gasoline-powered Pennsylvania-branded lawnmower that was the very same model I used to cut my family's lawn when I was about 12. First, I thought: "Why is the Smithsonian showing some dumb old lawnmower?" My second though: "Man, I must be geting old."

My first run-in with a computer was visiting my father's bank around 1960s where there was a monster tube-driven computer printing out punch-card checks in a vast refrigerated room. It was loud and printed out another check every couple of seconds, which was amazing back then. But that wouldn't be nearly as cool to see in the museum as this wall-hanger.

... and sell T-shirts that say "I'm living in the past, man". Or better yet, pocket protectors. Maybe we could join forces with the Society for Creative Anachronism and have mock sword fights with slide rules -- or use the giant ones as battering rams.

@Joe....nope, you're the man. SMS me your email address on 0428 425 099 and I'll get in touch (don't like posting my address directly). I can probably get it to Sydney for you to pick up (I'm in sunny Bathurst). Wife will be happy....something going out of the house instead of coming into it!

Eniac moved to Aberdeen, MD. after it was born, same as me! It lived at the BRL, where dad worked from the late 40s to the late 70s. I was a lucky little kid.

Ironically, we didn't get our own computer at home (a tragically tiny VIC, initially) until about 1980, although there were programmable TI 58s rattling around the house occasionally, which I learned to program for lack of anything better.

I learned to etch PCBs so I could expand the pitiful memory on that little VIC. Pitiful by today's standards, but relatively lavish compared to a TI58.